A knee-joint endoprosthesis can have a plate which is affixed to the tibia and which will hereinafter be referred to as a tibial plate and defines one or two sliding surfaces of a uniform character for engagement with the natural end of the femur or with a femoral part of the prosthesis which is affixed to the femur. When two such surfaces are provided, they respectively may constitute the lateral and medial surfaces.
In the case in which the knee-ligament apparatus of the patient remains intact or is comparatively healthy but the knee joint is damaged, a so-called sleigh-type prosthesis may be indicated. In this case, the femoral surfaces forming the knee joint are provided with skids along the condylar rolling or sliding regions, e.g. of steel, while a sliding surface is formed by a tibial plate which is affixed to the tibial.
The tibial plate may be used alone or in conjunction with a total prosthesis. The invention may also be used for knee-joint endoprostheses in which the ligaments have been damaged.
The natural tibial sliding surface is generally formed from solid bone material, namely, the corticalis, which is covered by a cartilage layer. For implantation of a tibial part, this rigid corticalis together with the usually damaged cartilage layer must be removed and the implant fixed on the tibial cross-sectional surface which is then formed. The tibial cross-sectional surface, of course, comprises a central region of spongiosa which constitutes the major part of the area of the bone and is surrounded by an edge region formed by the corticalis.
In the past, the tibial plate has been supported mainly, if not exclusively, on this corticalis shell.
The tibial plates hitherto used generally comprised a synthetic resin layer which formed the sliding surface and had the configuration at its upper side of the normal configuration of the upper end of the tibial plate before removal thereof to accommodate the implant. The underside of this synthetic resin layer was provided with a reinforcement which generally was a completely bending-resistant metal plate on the underside of which small fastening ribs were provided. These ribs could be received in grooves recessed in the spongiosa.
The implanted tibial plate is braced exclusively on the cross-sectional area contributed by the corticalis.
During a rolling and sliding action of the knee joint below the femoral skid or runner and the sliding surface of the tibial plate, all of the joint forces had to be taken up by the corticalis along the edge of the tibial plate.
The spongiosa surrounded by the supported edge region in this system takes up practically no force. As a result, the spongiosa tended to atrophy and recede so that below the metal back of the tibial plate, a cavity tended to form which was progressively filled by connective tissue which was comparatively soft. As a consequence, an attachment of the tibial plate in the region of the spongiosa tended to loosen to the detriment of the junction with the tibia as a whole. In the case of a weak corticalis or a corticalis which, as a result of the cavity formation, weakened with time, there was always the danger of breakage of the corticalis by overloading the knee joint and hence the need for reoperation.